The present invention relates in general to a method and apparatus for eliminating the effects of noise inherent in certain optical sources used in photoplethysmographic measurements.
In the science of photoplethysmography, light is used to illuminate or trans-illuminate living tissue for the purpose of measuring blood analytes or other hemodynamic or tissue properties. In this monitoring modality light is injected into living tissue and a portion of the light which is not absorbed by the tissues, or scattered in some other direction, is detected a short distance from the entry point. The detected light is converted into an electronic signal that is indicative of the received light signal from the tissue. This electronic signal is then used to calculate physiologic parameters such as arterial blood oxygen saturation and hemodynamic variables such as heart rate, cardiac output, or tissue perfusion. Among the blood analytes that may be measured by photoplethysmography are the various species of hemoglobin, including the percentages of oxyhemoglobin, carboxyhemoglobin, methemoglobin, and reduced hemoglobin in the arterial blood. A device which detects and processes photoplethysmographic signals to measure the levels of various blood analytes and hemodynamic parameters is referred to as a photoplethysmographic apparatus, device, or instrument. Typically these instruments also include, and control, the light sources or emitters used to generate the light that illuminates the tissue.
The first widespread commercial use of photoplethysmography in medicine was in the pulse oximeter, a device designed to measure arterial blood oxygen saturation. To make these measurements, two different bands of light must be used, with each light band possessing a unique spectral content. Each spectral band, or light band, is typically referred to by the center wavelength, or sometimes by the peak wavelength, of the given band. In pulse oximetry two different light emitting diodes (LEDs) are typically used to generate the sensing light, one with a center, or peak, wavelength near 660 nanometers (nm) and a second with a center, or peak, wavelength near 940 nm.
Light from each LED light source, or emitter, is passed into the tissue-under-test, usually a finger, earlobe, or other relatively thin, well-perfused tissue sample. After passing some distance through the tissue-under-test, a portion of the light not absorbed by the tissue or scattered in some other direction is collected by a photodetector and converted into electronic signals that are directly proportional to the received light signals. The channels, or electronic signals from each of the different light sources, are kept separated through the use of any one of a number of different well-published techniques, including but not limited to, time-division multiplexing or frequency-division multiplexing.
The signals received from the tissue are referred to as photoplethysmographic signals. These signals consist of a small pulsatile component and a rather large constant component that changes slowly over time when compared with the pulsatile component of the signal. The pulsatile component of the photoplethysmographic signal is created by the pulsation of the blood in the tissue-under-test. When the heart contracts, it pushes blood out of the heart and into the peripheral tissues. This increases the optical density of the tissue located between the emitter and detector elements of the sensor, which decreases the amplitude of the received optical signals. As the heart relaxes and refills with blood, the optical density of the tissue-under-test decreases, and the received signal amplitude increases. The comparatively constant component of the photoplethysmographic signal is often referred to as the DC component of the signal, and the pulsatile component of the photoplethysmographic signal is often referred to as the AC component of the signal.
The photoplethysmographic signals are processed to obtain a measurement of the oxygen saturation in the arterial blood. This can be done in a number of different ways but all require mathematically relating the amplitude of the photoplethysmographic signals from each of the two channels to the arterial oxygen saturation.
In conventional pulse oximetry the instrument has only two channels, one associated with each emitter or light source used. With only two channels, only two blood analytes can be measured. Conventional pulse oximetry makes the mathematical assumption that there are primarily only two types of blood analytes in the arterial blood, oxyhemoglobin and reduced hemoglobin.
In order to measure only the arterial oxygen saturation, the pulse oximeter makes use of both the pulsatile component and the DC component of the photoplethysmographic signals. Because any pulsation of the venous system or capillaries is small by comparison to the arterial pulsation, changes in the amplitude of the photoplethysmographic signals will be dominated by the arterial pulsation. Note that the photoplethysmographic signals can be severely distorted by artifacts such as patient motion or electrocautery but elimination of these sources of artifacts is not the focus of this patent and will not be specifically addressed herein.
The amplitude of the pulsatile component of these photoplethysmographic signals can be extremely small. It is not uncommon for the percent modulation, or the peak-to-peak amplitude of the pulsatile portion divided by the constant portion, to be less than one part in one thousand, or 0.1%. Thus it is crucial that extremely quiet light sources are used to generate the signals for probing the tissue-under-test. This is necessary because any intensity noise in the light source that is within the frequency range of the passband of the photoplethysmographic device, or which can alias into its passband, will show up in the received photoplethysmographic signals and corrupt the desired measurements. Thus to allow for accurate and precise photoplethysmographic measurements, the light sources should be extremely quiet (also referred to as xe2x80x9cnoise freexe2x80x9d) or a means must be found to eliminate the light noise from the received signals.
Since the inception of photoplethysmography, this monitoring modality has been used to detect more and more different parameters. For example, a device was disclosed in Jarman et al U.S. Pat. No. 5,983,122 that is capable of measuring the percentages of four different analytes in the arterial blood, including oxyhemoglobin, carboxyhemoglobin, methemoglobin, and reduced hemoglobin.
As the number of different parameters measured by photoplethysmography increases, so too does the number of different bands of light required to make the measurements. Further, because a fairly high intensity of light over a fairly narrow spectral range is needed for these measurements, it has been found that the most successful sources of light for these measurements have been discrete, narrow-band emitters such as LEDs or laser diodes. These types of light sources are typically used because broadband sources (in conjunction with filters or a diffraction grating to obtain the required spectral bands) produce too little energy over the desired narrow spectral bandwidths to provide sufficient signal amplitude for photoplethysmographic measurements.
LEDs are inherently very quiet light sources but do not have a sufficiently narrow spectral bandwidth for use at all of the required center wavelengths. Conventional edge-emitting diode lasers provide the necessary narrow bandwidth but can be quiet noisy. For example, in a time-division multiplexed system, the intensity of the light emitted by a laser diode can vary from pulse to pulse, or can even jump almost instantaneously during any given pulse. These intensity variations can easily be large enough to prevent the measurement of the desired blood analytes to clinically-acceptable accuracy and precision levels. While not all types of laser diodes are noisy, often the photoplethysmographic instrument designer must use an inherently noisy light source if it is the only one available that has the required bandwidth and center wavelength needed for measurement of the desired blood analytes.
Vertical cavity surface-emitting lasers (VCSELs) can be very quiet but are generally unavailable at wavelengths shorter than about 800 nm. Thus for the shorter wavelengths it is often necessary to use an inherently noisy edge-emitting laser diode. Unfortunately, the noise levels in this type of laser have made the development of commercially viable photoplethysmographic devices based on these types of light sources impossible as the magnitude of the laser noise simply introduces too much error into the desired analyte measurements.
One of the reasons for the noise in the output of edge-emitting lasers is that once the laser is energized, the semiconductor junction temperature increases and the length of the laser cavity begins to change due to thermal expansion of the semiconductor die. If the cavity length changes, the original lasing wavelength of the device will no longer be the optimal wavelength for the new cavity length, because it will not fit inside the cavity with a whole number of wave periods. When this occurs a new lasing wavelength will become dominant within the cavity, essentially xe2x80x9ccrowding outxe2x80x9d the wavelength that was previously lasing. When this transition occurs there is also a small instantaneous change in the output optical power. It is these sudden changes that cause the noise that is seen at the output of these noisy light sources.
Mode hopping is only one source of light intensity noise. Other types of emitters can introduce noise as well. Tungsten light sources, for example, can be noisy just due to movement of the tungsten filament in the bulb. This type of broadband light source has been used in the past in photoplethysmographic instrumentation. To obtain the necessary spectral content the light from this element can be filtered by narrowband filters or spectrally dispersed by a diffraction grating. In either case the photoplethysmographic signals derived from these types of light sources can have high enough noise levels to cause large inaccuracies in the desired measurements.
It is important to recognize that as the number of blood analytes to be measured by a single photoplethysmographic device increases, the number of light sources (or at least the number of channels) also increases, and the effects of even very small amounts of noise are more and more noticeable. This is directly attributable to the fact that each photoplethysmographic signal, originating from each emitter, is simultaneously reading multiple blood analytes. As a result the light levels of each channel must be read with higher accuracy, and the detrimental effect of any given amount of noise on that channel is greater.
It has long been recognized by the medical community that conventional pulse oximetry is inaccurate in the presence of additional species of hemoglobin beside oxyhemoglobin and reduced hemoglobin. Steven J Barker, M.D., in a 1987 article entitled xe2x80x9cThe Effect of Carbon Monoxide Inhalation on Pulse Oximetry and Transcutaneous P02,xe2x80x9d published in the journal Anesthesiology, explained the errors in pulse oximetry caused by elevated levels of carboxyhemoglobin. Two years later in another Anesthesiology article entitled xe2x80x9cEffects of Methemoglobinemia on Pulse Oximetry and Mixed Venous Oximetry,xe2x80x9d Barker defined the errors in pulse oximetry readings caused by elevated methemoglobin levels. These articles, along with numerous case studies, make clear the long-standing need for a pulse oximeter capable of measuring all four primary species of hemoglobin. Additionally, the Pologe patent, U.S. Pat. No. 5,891,022, from 1999, the Pologe et al patent, U.S. Pat. No. 5,790,729, from 1988, and the Barthelemy et al patent U.S. Pat. No. 5,413,100 from 1995, all recognized the need for the use of multiple laser diodes to measure the additional blood analytes discussed above. These patents demonstrate the understanding that multiple laser diode based emitters, with carefully selected center wavelengths, must be combined to create an instrument capable of measuring the four primary hemoglobin species.
Despite the long-standing recognition of the clinical need for this type of instrument no photoplethysmographic device exists to measure these four species of hemoglobin. The primary problem that continues to prevent these ideas from developing into a commercially viable instrument is the high noise level inherent in most commercially available laser diodes.
In the design of a multi-parameter photoplethysmographic device, it is necessary to use emitters that have narrow bandwidths and mathematically-selected center wavelengths to allow for the measurement of the desired blood analytes. This can necessitate the use of inherently noisy light sources. Unfortunately these noisy sources distort the very photoplethysmographic signals that are necessary for these measurements. Thus it is necessary to find a way to utilize these noisy sources and still make analyte measurements to the required levels of accuracy and precision.
In the science of photoplethysmography, light is used to illuminate, or trans-illuminate, tissue for the purpose of measuring blood analytes or hemodynamic properties or parameters. In making these measurements it can become necessary to use light from a number of different types of sources including, but not limited to, incandescent bulbs, light-emitting diodes (LEDs), and lasers. The use of laser light sources becomes necessary when very narrow spectral bandwidth (narrow band) light is required to make possible the accurate photoplethysmographic measurement of certain specific blood analytes or the simultaneous measurement of a number of blood analytes such as oxyhemoglobin, reduced hemoglobin, carboxyhemoglobin, and methemoglobin. It is the purpose of this invention to allow accurate photoplethysmographic measurements using inherently noisy light sources that otherwise have desirable optical properties, including precisely tunable or selectable center wavelengths and high output intensity over a narrow spectral bandwidth.
This invention combines the use of at least one relatively quiet optical source with any number of noisier optical sources. For example, in one embodiment of this invention, a light emitting diode (LED) is used in combination with a number of edge-emitting laser diodes to provide all of the required spectral bands (of light) necessary to measure the target analytes. In this example, the targeted blood analytes include oxyhemoglobin, reduced hemoglobin, carboxyhemoglobin, and methemoglobin. The laser diodes provide high-intensity, narrowband, albeit relatively noisy, outputs at the center wavelengths that are required to allow measurement of these blood analytes. The LED provides an optically stable (also referred to as quiet or clean) output that is used, at least in part, to minimize the effect of the noise that is inherent in the laser sources.
As any noise in the input light sources becomes noise in the photoplethysmographic waveforms that are received by the photodetector, this intensity noise distorts the waveforms used to determine the blood analyte levels and can dramatically increase the inaccuracy of the measurements. Note also that if there is a reduction in the patient""s perfusion at the sensor site (the position on the tissue where the sensing light enters and is received from the tissue-under-test), there is a dramatic increase in the errors in the analyte measurements because although there is a fixed amount of intensity noise inherent in the light sources, this noise becomes larger in proportion to the pulsatile component of the photoplethysmographic waveform. This invention provides a way to utilize a quiet (or relatively quiet) light source to eliminate or at least minimize the effects of light intensity noise generated by inherently noisy (or relatively noisy) light sources in a photoplethysmographic device.
In this invention a quiet source, in this example the LED, provides a quiet photoplethysmographic waveform which acts as a xe2x80x9ctemplatexe2x80x9d that is used to eliminate, or at least minimize, the noise inherent in the photoplethysmographic waveforms generated by the noisier sources.
In an ideal photoplethysmographic device, different light sources emit light that is incident on the tissue-under-test through essentially the same small output aperture. The light from each source then ideally passes through the tissue traversing the same optical path. This generates a set of photoplethysmographic waveforms, one for each emitter, which are received by the detector, typically a silicon photodiode or equivalently any type of photodetector that is sensitive to the wavelengths of light emitted by the sources. The light signals from the different sources are kept separate from each other by any one of a number of different electronic schemes such as time-division multiplexing, where only one light source is turned on at a time and all the emitters are cycled through in rapid succession. Another scheme sometimes used is frequency-division multiplexing, where each light source is modulated at a different frequency. These are well-defined techniques in the art of electronics in general which have both been extensively used in the past in photoplethysmography, and will thus not be further explained herein.
In a perfectly noise free system, each photoplethysmographic waveform can be mapped into any other waveform by nonlinear amplitude scaling. In photoplethysmography the transform that scales one waveform into another is known; what is not known is the magnitude of the scaling that is required, as this depends on the level of the blood analytes and other absorbers in the tissue-under-test.
If the intensity noise generated by the noisy light sources is random in nature, a statistical least squares fit of the waveform from the quiet light source will allow the correct selection of the scale factors required to create a noise free (or at least quiet) version of the waveform data originating from any of the noisy light sources. This set of quiet photoplethysmographic waveforms can then be used in the calculation of the various desired blood analytes or hemodynamic variables. In this way the clean (or quiet) photoplethysmographic waveform has acted as a template to provide the correct waveshape for the noisy waveforms. This allows the use of noisy sources in the photoplethysmographic measurements, while eliminating or at least minimizing the error that would have been generated by the intensity noise.
The quiet light source used in the instrument may serve a dual role. It may provide the quiet light source for minimizing the effects of the noise in the noisy channels and it may also be one of the selected center wavelengths, or spectral bands, used for the analyte or hemodynamic measurements. The advantage of this is that the dual use of the quiet channels reduces the number of light sources required for the system, thus minimizing the complexity and cost of the instrument. In reduction to practice at the current time, the optimal light sources for the quiet channels will typically be either LEDs or VCSELs. The term xe2x80x9cchannelxe2x80x9d is used to indicate those data associated with the light out of the tissue-under-test, picked up by the sensor photodetector, originating from any given emitter. These data can be in the form of light levels, current levels, voltage levels, or mathematical values after conversion of the analog signals to digital values.
In this patent the term xe2x80x9ctemplatexe2x80x9d is used in conjunction with any technique in which the data from the quiet channel (or channels) is utilized to minimize the effects of the noise in the noisy channels. The template is the clean, or relatively noise free, photoplethysmographic data. The conceptually simplest technique, where the waveshape of the quiet channel is scaled to fit the data from the noisy channels, is explained above. This is just one of a number of ways of performing the noise elimination using a known clean channel. A second method, utilizing linear regression of differential absorption measurements, is described in detail below. While the two techniques are mathematically different, they both use the quiet channels to clean up the noisy ones, and therefore the quiet channel is still considered to be a template.